1. Selling errors on the dial;
2. The logo may not be centred properly;
3. On a metal strap the links do not move freely;
4. Letters printed on the dial are not sharp and crisp;
5. Functions like sub dials and helium release values do not function;
6. The weigh less because of the cheaper materials used;
7. Lack of or Poor packaging;
8. A loud ticking noise, most quality watches are very quiet;
9. Fingerprints, dust or other residue inside the watch.

In most cases, you can spot fakes based on the movements. One should avoid buying a watch without seeing the movement. This eliminates most of the fakes out there because most don't bother trying to replicate the movement's look .

As an aside, the weight of a watch is not a good marker of authenticity. A fake Rolex can weight as much as or more than a real one.